At the time my van was the heaviest Jeff had worked with (11.7). He should have it dialed in by now, just weigh the axles.
To me the difference off road is that the Deaver Springs make me feel like I'm aired down to 35 pounds.
I've posted before about the switch from SMB's 2006 springs to the Deaver's. Every time I do some kind of upgrade here comes all the extra crap. I now expect it.
>>Much smoother. I can drive over some of the worst speed bumps and not feel much.
>>Gained 2+ inches on the lift.
>>Interior doesn't vibrate apart like it did in the past.
>>Got better shocks out of the deal.
<<Additional lift made it difficult to get into my van. Had to buy power steps
<<Made my brand new 285's look stupid on the van and had to go with 315's
<<Had to cut back the front fenders because with the sway bar cut loose the tires try to eat the flares
<<The van doesn't take cornering as well. Even with the 285's the springs just allow a bit more sway. The 315's made it a bit worse.
I would never go back to what SMB originally installed but other spring packs might do as well...I don't know. I am happy with how it handles with the Deaver's overall. I do think each van might be different.
The weight and length of the van plus the rim and tire size all come into play. I'm also sure the year of the van is something to consider as well.
Jeff had to send me a couple of extra leafs for the rear because it had a slight sag. Adding the leaf didn't change the ride. I also put my rear airbags back on and with them aired to 35 pounds, the ride is the same.
On the older vans it might be wise to ask Jeff about front shock mounts. Some need gussets added. At least on mine the weld was no good on one and failed in the backcountry.